In one embodiment of shearing interferometry, a measurement mask is positioned in the object plane of the optical system to be tested, a measurement structure in the form of a coherence mask being arranged on the measurement mask. The coherence mask can be embodied e.g. as a diffraction grating and serves to split illumination radiation incident on the coherence mask into various individual rays that pass through the projection lens on different paths. An analysis grating is situated in the image plane of the imaging system. The superimposition of the waves generated by diffraction at the analysis grating gives rise to a superimposition pattern in the form of an interferogram, which is detected with the aid of a suitable detector. Possible embodiments of a coherence mask and of a diffraction grating of a shearing interferometer are specified, for example, in DE 10 2005 041 373 A1.
An optimum measurement result is not always achieved since often it is not ensured that the individual rays generated by the coherence mask have an angular distribution that optimally fills the pupil of the projection lens. In other words, it is not always ensured that the intersection points of the paths of the individual rays with the pupil plane extend as closely meshed as possible over as far as possible the entire area of the pupil.